[PATCH v2] add -Wpointer-arith sparse flag to toggle sizeof(void) warnings

[PATCH v2] add -Wpointer-arith sparse flag to toggle sizeof(void) warnings

[Joey Pabalinas]

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Recent changes to the min()/max() macros in include/linux/kernel.h
have added a lot of noise when compiling the kernel with Sparse checking
enabled. This mostly is due to the *huge* increase in the number of
sizeof(void) warnings, a larger number of which can safely be ignored.

Add the -Wpointer-arith flag to enable/disable these warnings (along
with the warning when applying sizeof to function types exactly like the
GCC -Wpointer-arith flag) on demand; the warning itself has been disabled
by default to reduce the large influx of noise which was inadvertently
added by commit 3c8ba0d61d04ced9f8 (kernel.h: Retain constant expression
output for max()/min()).

Update the manpage to document the new flag.

CC: Kees Cook <keescook@chromium.org>
CC: Linus Torvalds <torvalds@linux-foundation.org>
CC: Martin Uecker <Martin.Uecker@med.uni-goettingen.de>
CC: Al Viro <viro@ZenIV.linux.org.uk>
CC: Christopher Li <sparse@chrisli.org>
CC: Joey Pabalinas <joeypabalinas@gmail.com>
CC: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Joey Pabalinas <joeypabalinas@gmail.com>
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>

 4 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/evaluate.c b/evaluate.c
index 4e1dffe9c5416380df..f828da37df8e686623 100644
--- a/evaluate.c
+++ b/evaluate.c
@@ -2193,7 +2193,8 @@ static struct symbol *evaluate_sizeof(struct expression *expr)
 	size = type->bit_size;
 
 	if (size < 0 && is_void_type(type)) {
-		warning(expr->pos, "expression using sizeof(void)");
+		if (Wpointer_arith)
+			warning(expr->pos, "expression using sizeof(void)");
 		size = bits_in_char;
 	}
 
@@ -2204,7 +2205,8 @@ static struct symbol *evaluate_sizeof(struct expression *expr)
 	}
 
 	if (is_function(type->ctype.base_type)) {
-		warning(expr->pos, "expression using sizeof on a function");
+		if (Wpointer_arith)
+			warning(expr->pos, "expression using sizeof on a function");
 		size = bits_in_char;
 	}
 
diff --git a/lib.c b/lib.c
index 73d372c36626538bab..f7fdac96674aec4c24 100644
--- a/lib.c
+++ b/lib.c
@@ -242,6 +242,7 @@ int Woverride_init = 1;
 int Woverride_init_all = 0;
 int Woverride_init_whole_range = 0;
 int Wparen_string = 0;
+int Wpointer_arith = 0;
 int Wptr_subtraction_blows = 0;
 int Wreturn_void = 0;
 int Wshadow = 0;
@@ -654,6 +655,7 @@ static const struct flag warnings[] = {
 	{ "return-void", &Wreturn_void },
 	{ "shadow", &Wshadow },
 	{ "sizeof-bool", &Wsizeof_bool },
+	{ "pointer-arith", &Wpointer_arith },
 	{ "sparse-error", &Wsparse_error },
 	{ "tautological-compare", &Wtautological_compare },
 	{ "transparent-union", &Wtransparent_union },
diff --git a/lib.h b/lib.h
index 3050b5577ba4d42e97..e34bb9a02ebac03f52 100644
--- a/lib.h
+++ b/lib.h
@@ -151,6 +151,7 @@ extern int Woverride_init;
 extern int Woverride_init_all;
 extern int Woverride_init_whole_range;
 extern int Wparen_string;
+extern int Wpointer_arith;
 extern int Wptr_subtraction_blows;
 extern int Wreturn_void;
 extern int Wshadow;
diff --git a/sparse.1 b/sparse.1
index 88343f3170f195297a..4379406999c94b806e 100644
--- a/sparse.1
+++ b/sparse.1
@@ -288,6 +288,25 @@ Standard C syntax does not permit a parenthesized string as an array
 initializer.  GCC allows this syntax as an extension.  With
 \fB\-Wparen\-string\fR, Sparse will warn about this syntax.
 
+Sparse does not issue these warnings by default.
+.
+.TP
+.B \-Wpointer-arith
+Warn about anything that depends on the \fBsizeof\fR a function type or of void.
+
+C99 does not allow the \fBsizeof\fR operator to be applied to function types or to
+incomplete types such as void. GCC allows \fBsizeof\fR to be applied to these
+types as an extension and assigns these types a size of \fI1\fR.
+
+Although non-standard (and somewhat illogical), constructs such as \fBsizeof(void)\fR
+are often useful when the intent is to operate on an expression without evaluating
+it, e.g. in the following integer constant expression predicate:
+
+.nf
+#define __is_constexpr(x) \\
+	(sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))
+.fi
+
 Sparse does not issue these warnings by default.
 .
 .TP
-- 
2.17.0.rc1.35.g90bbd502d54fe92035.dirty

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Re: [PATCH v2] add -Wpointer-arith sparse flag to toggle sizeof(void) warnings

[Linus Torvalds]

Ack, looks sane.

             Linus

Re: [PATCH v2] add -Wpointer-arith sparse flag to toggle sizeof(void) warnings

[Luc Van Oostenryck]

On Sat, Apr 07, 2018 at 11:58:05AM -1000, Joey Pabalinas wrote:
> Recent changes to the min()/max() macros in include/linux/kernel.h
> have added a lot of noise when compiling the kernel with Sparse checking
> enabled. This mostly is due to the *huge* increase in the number of
> sizeof(void) warnings, a larger number of which can safely be ignored.
> 
> Add the -Wpointer-arith flag to enable/disable these warnings (along
> with the warning when applying sizeof to function types exactly like the
> GCC -Wpointer-arith flag) on demand; the warning itself has been disabled
> by default to reduce the large influx of noise which was inadvertently
> added by commit 3c8ba0d61d04ced9f8 (kernel.h: Retain constant expression
> output for max()/min()).

Thanks. 
It's good to me. I just have a few remarks/suggestions:
 
With the warning disabled by default (for the moment), I had to adapt
the testsuite with:

diff --git a/validation/sizeof-function.c b/validation/sizeof-function.c
index 27d535d4e..8ff67f214 100644
--- a/validation/sizeof-function.c
+++ b/validation/sizeof-function.c
@@ -35,7 +35,7 @@ int test(void)
 
 /*
  * check-name: sizeof-function
- * check-command: sparse -Wno-decl $file
+ * check-command: sparse -Wpointer-arith -Wno-decl $file
  *
  * check-error-start
 sizeof-function.c:22:14: warning: expression using sizeof on a function

> diff --git a/sparse.1 b/sparse.1
> index 88343f3170f195297a..4379406999c94b806e 100644
> --- a/sparse.1
> +++ b/sparse.1
> @@ -288,6 +288,25 @@ Standard C syntax does not permit a parenthesized string as an array
>  initializer.  GCC allows this syntax as an extension.  With
>  \fB\-Wparen\-string\fR, Sparse will warn about this syntax.
>  
> +Sparse does not issue these warnings by default.
> +.
> +.TP
> +.B \-Wpointer-arith
> +Warn about anything that depends on the \fBsizeof\fR a function type or of void.

Maybe it would be useful to add something along the line of "like directly using
the sizeof operator on void or doing pointer arithmetic on a void pointer" ?

> +Although non-standard (and somewhat illogical), constructs such as \fBsizeof(void)\fR
> +are often useful when the intent is to operate on an expression without evaluating
> +it, e.g. in the following integer constant expression predicate:
> +.nf
> +#define __is_constexpr(x) \\
> +	(sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))
> +.fi

I think that pointer arithmetic is much more useful than taking the size of void
(being able to take the size of *any* thing is somewhere in the middle, IMO).
But in all case, I don't think this part should belong to the man page.

Cheers,
-- Luc

Re: [PATCH v2] add -Wpointer-arith sparse flag to toggle sizeof(void) warnings

[Joey Pabalinas]

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On Sun, Apr 08, 2018 at 09:48:24AM +0200, Luc Van Oostenryck wrote:
> With the warning disabled by default (for the moment), I had to adapt
> the testsuite with:

Ah, so should I include that change in the patch itself when I make a V3?

> > +Warn about anything that depends on the \fBsizeof\fR a function type or of void.
> 
> Maybe it would be useful to add something along the line of "like directly using
> the sizeof operator on void or doing pointer arithmetic on a void pointer" ?

I actually just took the explanation straight from the GCC man page
since I figured the explanation should match (as the flag itself is
basicallt copied).

But I do sort of like your wording of it more, so if no one else sees
any reasons to _not_ to diverge from GCC's wording here I have no problem
changing that.

> > +Although non-standard (and somewhat illogical), constructs such as \fBsizeof(void)\fR
> > +are often useful when the intent is to operate on an expression without evaluating
> > +it, e.g. in the following integer constant expression predicate:
> > +.nf
> > +#define __is_constexpr(x) \\
> > +	(sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))
> > +.fi
> 
> I think that pointer arithmetic is much more useful than taking the size of void
> (being able to take the size of *any* thing is somewhere in the middle, IMO).
> But in all case, I don't think this part should belong to the man page.

Also have no problem eliding this section if no one else has any
good arguments for keeping it.

-- 
Cheers,
Joey Pabalinas

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Re: [PATCH v2] add -Wpointer-arith sparse flag to toggle sizeof(void) warnings

[Joey Pabalinas]

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On Mon, Apr 09, 2018 at 12:51:01PM -1000, Joey Pabalinas wrote:
> Ah, so should I include that change in the patch itself when I make a V3?

Sending a v3 now incorporating the suggestions.

-- 
Cheers,
Joey Pabalinas

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